EDP Sciences

Vol. 623
In section 4. Extragalactic astronomy

Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of the Swift/BAT6 complete sample of bright LGRBs. III. Stellar masses, star formation rates, and metallicities at z > 1

by J. T. Palmerio, S. D. Vergani, R. Salvaterra, et al. 2019, A&A, 623, A26 alt

Long gamma-ray bursts (LGRB, which emit for periods of more than 2 sec.) are connected to the end of life of massive stars, which explode in supernovae. Since those massive stars are short lived, LGRBs are associated with the recent (last 10 Myr) star formation rate (SFR) of a galaxy. Could LGRBs then be used as tracers of SFR? This is debatable, since previous studies at z <1 revealed that only low-metallicity environments are conducive to the production of LGRB. The present work tackles this issue with a complete study of LGRB host galaxies at redshifts between one and two. The authors compare their stellar mass, star formation rate, and metallicity with a control sample of typical star-forming galaxies within the same redshift range. They find that, similarly to z <1, LGRBs do not directly trace star formation at 1 < z < 2: while they can sometimes host starbursts, the determinant parameter for a galaxy to possess an LGRB is metallicity. Only when a galaxy sample is restricted to low metallicity, meaning 12 + log O/H lower than 8.55 (or 0.7 Zsolar), LGRBs can be used as a tracer of SFR. This metallicity threshold found (0.7 Zsolar) may appear high, but corresponds to the conditions necessary to rapidly produce a fast-rotating Wolf-Rayet stars in close binary systems, and could be accommodated by single star models under chemically homogeneous mixing with very rapid rotation and weak magnetic coupling. Since the average metallicity of galaxies decreases with redshift, it becomes possible at z> 3, to have a sample of galaxies below this critical metallicity. LGRBs, therefore, become good tracers of star formation at z > 3.